Bob aptly describes the COVID-era rules and restrictions. The cruise lines' first priority is the safety of their clients and crew. In addition, the towns where cruise lines dock are also sensitive to a weekly surge of "new people" in the area. And hence, one result is the shore excursion restrictions as Bob described. Hopefully, in these COVID times, your cruise will sail as scheduled - and even better - these COVID restrictions won't be necessary and you can travel freely when going ashore. But until then, these COVID rules will apply. We've all seen the stories - even recently - of infections occurring aboard a cruise ship even when everyone boarded with a negative test.
I have sailed with Princess often and they do provide detailed Terms & Conditions. If you bought the cruise from a Travel Agent, Online Vendor, Costco, etc, it is the responsibility of the seller to provide you these terms and conditions. In addition, Princess also has a COVID-19 - Cruise with Confidence document that provides information and procedures about your health, safety, and well-being. And lastly, some cruises are now adding a COVID waiver.
These are strange times and everyone is working to provide the "best" options for everyone - cruisers, crew, and local folks in the arrival ports. And until there's not a COVID threat, cruising - and travel - will be different. I would guess that Princess, like all cruise lines, would address these restrictions in their T&C or secondary documentation at the time of deposit of finalized booking.